Resonant converters such as LLC resonant converters are widely used in bus converter where no stringent load regulation is required. The resonant converter typically works with open loop control, and is designed to operate at a switching frequency fs according to the following equation (1) for best efficiency performance.
                              f          s                =                  1                      2            ⨯            Π            ⨯                          (                                                Lr                  ⨯                  Cr                                +                Tdead                            )                                                          (        1        )            
wherein Lr represents the inductance of a leakage inductor, Cr represents the capacitance of a resonant capacitor, Tdead represents the dead time between a high-side power switch and a low-side power switch in a power stage.
This switching frequency works well in heavy load condition. However, in light load condition, the primary current and the secondary current both decrease, so the conduction loss is low. But the driving loss and the switching loss are same as they are in the heavy load condition, which leads to low efficiency in light load condition.